This application relates generally to insulating glass units and, more particularly, to an insulating glass unit having a dual-seal system that provides good protection against moisture vapor permeability as well as improved structural integrity.
Insulating glass (IG) units are used in a wide variety of applications, such as skylights, high temperature environment viewing windows, and architectural windows, just to name a few. IG units are typically utilized to reduce heat transfer, such as between the inside and outside of a building.
A typical IG unit is formed by two glass sheets separated near their edges by a spacer to provide a chamber between the two glass sheets. This chamber is typically filled with a selected insulating atmosphere, such as argon, to enhance the insulating characteristics of the IG unit. A sealant system is used to bond the two glass sheets to the spacer. The sealant system is expected to provide sufficient structural strength to maintain the unity of the IG unit and also to provide sufficient protection against the insulating atmosphere leaking out of the chamber and/or moisture vapor in the ambient atmosphere outside the IG unit from moving into the chamber. Examples of conventional IG units are disclosed in U.S. Pat. Nos. 4,193,236; 4,464,874; 5,088,258; and 5,106,663; and European reference EP 65510, the disclosures of which are herein incorporated by reference.
The strength and performance of the IG unit depend heavily upon the sealant system and type of sealant used to secure the glass sheets to the spacer. The majority of sealants currently in use may be divided generally into two major types: (1) xe2x80x9cstructural sealantsxe2x80x9d and (2) xe2x80x9clow moisture vapor transmission (MVT) rate sealantsxe2x80x9d.
Structural sealants form a covalent chemical bond between the glass sheet and the spacer and promote the structural integrity of the IG unit. Examples of structural sealants include thermoset materials, such as polysulfides, polyurethanes, and silicone. These thermoset materials typically have a relatively high xe2x80x9cmodulusxe2x80x9d. As will be understood by one of ordinary skill in the IG unit art, the term xe2x80x9cmodulusxe2x80x9d relates to the stress/strain relationship of a material, i.e., the force required to stretch or elongate a material a certain distance. The modulus is conventionally defined as the slope of the stress/strain curve for a material and may be calculated in accordance with ASTM D412. The higher the modulus value, the more force which is required to elongate or stretch the material, i.e., the stronger is the material. Polyurethane, polysulfide, and silicone thermoset materials typically have modulus values in the range of several hundred psi. While enhancing the structural integrity of the IG unit, structural sealants typically provide poor MVT characteristics, e.g., 10 g/m2/day or greater (as measured in accordance with ASTM F1249), and also provide relatively high gas transmission rates. For example, polyurethane, polysulfide, and silicone materials typically have MVT rates in the range of about 15, 25, and 50 g/m2/day, respectively. As a result, IG units made only with conventional structural sealants do not typically provide commercially acceptable MVT characteristics or gas retention properties.
On the other hand, low MVT sealants, which do not covalently bond to the glass sheets and/or the spacer, provide improved MVT characteristics, e.g., less than 10 g/m2/day, and improved gas barrier capabilities compared to structural sealants but provide poorer structural integrity. Examples of low MVT sealants include thermoplastic materials, such as hot-melt materials, e.g., polyisobutylene (PIB). PIB materials typically have an MVT value of about 1.0 g/m2/day or less.
Also, thermoplastic hot-melt sealants typically must be applied at temperatures exceeding 300xc2x0 F. (149xc2x0 C.). This high temperature requirement may result in increased manufacturing costs due to higher energy consumption and the need for specialized, high-temperature equipment. Additionally, these thermoplastic materials typically have a lower modulus than thermoset materials, i.e., the thermoplastic materials require less force to stretch or elongate and have a tendency cold-flow. For example, PIB has a modulus value of about 30 psi (2.1 kg/cm2). Therefore, thermoplastic sealants are subject to softening when exposed to heat and, when placed under load, can flow or deform excessively to relieve the load. As a result, IG units made only with conventional thermoplastic sealants typically do not provide commercially acceptable structural characteristics.
A problem with using a single sealant for an IG unit having a conventional rigid spacer arises from the sealant thickness differences in the sealant system. For example, the thickness (width) of the sealant between the side of the spacer and the adjacent glass sheet (side region) is much less than the thickness of the sealant located between the glass sheets outside of the spacer (outer region). Therefore, if one of the glass sheets moves outwardly from the spacer, for example due to a change in atmospheric pressure, the relative percent of elongation for the thinner sealant portion in the side region is much larger than that for the thicker sealant portion in the outer region. This means that the thinner sealant portion in the side region is carrying practically all of the load of the sealant-system, which may cause this sealant portion to split or fail prematurely.
Recently, attempts have been made to develop xe2x80x9chybridxe2x80x9d sealants for single sealant IG units that have the low MVT characteristics of a thermoplastic material with the structural characteristics of a thermoset material. For example, U.S. Pat. No. 5,849,832 discloses a one component sealant combining a thermoplastic hot-melt resin blended with an atmospheric curing polymer. The MVT characteristics of this sealant, e.g., about 3.0-4.0 g/m2/day, are better than the MVT characteristics of conventional thermoset sealants but are still higher than for thermoplastic sealants, such as PIB. Additionally, since this sealant provides the IG unit with structural integrity, it has a modulus of about 250 psi (17.5 kg/cm2). Further, this material is harder than conventional thermoplastic materials, e.g., has an initial hardness greater than about 50 Shore A and a cured hardness of greater than about 60 Shore A (as measured in accordance with Sealed Insulating Glass Unit Manufacturers Association (SIGMA) test procedure P.1.A. using a Shore gauge (scale A) commercially available from the Shore Instrument Company). Therefore, this material does not completely overcome the drawbacks discussed above.
As an alternative to single sealant systems, so-called xe2x80x9cdual-sealxe2x80x9d systems were developed to combine the relative advantages of structural sealants and low MVT sealants. A conventional dual-seal system utilizes a low MVT thermoplastic inner or primary sealant located primarily on the side region of the spacer to reduce moisture vapor transmission into the chamber. This primary sealant provides little or no structural integrity to the IG unit. A secondary, outer structural thermoset sealant is located primarily on the outside of the spacer (outer region) to bond the spacer and glass sheets together to provide the IG unit with structural integrity.
However, even in these dual-seal systems, under normal use there is a natural tendency for the outside edges of the glass sheets to rotate or flex due to changes in atmospheric pressure, temperature, wind load, or altitude changes. Under these circumstances, the thermoplastic primary sealant tends to expand and contract and may pull away from the glass sheet and/or spacer. This may cause gaps in the sealant system through which moisture may enter the chamber or through which the insulating atmosphere may leak out of the chamber.
Therefore, it would be advantageous to provide a dual-seal system for an IG unit which provides low MVT characteristics but which also provides improved structural performance over conventional sealant systems. It would also be desirable if the primary sealant of the sealant system possessed a lower modulus value than conventional structural sealants or hybrid sealants to reduce the stress typically carried by primary sealants located on the side region of an IG unit.
An insulating glass unit of the invention comprises a first pane of glass, a second pane of glass, and a spacer system. The spacer system comprises (i) a spacer positioned between an inner surface of the first pane of glass and an inner surface of the second pane of glass and (ii) a sealant system for adhering the inner surfaces of the glass panes to the spacer. The sealant system comprises a sealant comprising (a) at least one thermoplastic hot-melt material having a melt temperature ranging from about 125xc2x0 F. (52xc2x0 C.) to about 250xc2x0 F. (121xc2x0 C.), and (b) at least one curable material. The sealant, when cured, forms a covalent bond between the spacer and the panes. The sealant has an initial hardness ranging from about 25 Shore A to about 45 Shore A and a post-cure hardness measured about 48 hours thereafter ranging from about 30 Shore A to about 50 Shore A.
Another insulating glass unit comprises a first pane having an inner surface and an outer surface and a second pane having an inner surface and an outer surface, with the panes positioned such that the inner surface of the first pane faces the inner surface of the second pane. A spacer is located between the first and second panes and a sealant system adheres the panes to the spacer. The sealant system comprises (a) a first sealant comprising a thermoplastic material and a curable material, and (b) a second sealant. The first sealant has a moisture vapor transmission rate of less than about 2.5 g/m2/day and a hardness after curing ranging from about 30 Shore A to about 50 Shore A.